Calcium carbonate is the major constituent of limestone and occurs naturally in two crystalline forms, aragonite and calcite. On heating limestone under controlled calcination conditions, carbon dioxide is driven off leaving calcium and magnesium oxide, otherwise known as quicklime. The effluent carbon dioxide gas has often been associated with the treatment of waste residues from paper manufacturing processes. The discharges from most pulping and paper making processes have a high level of caustic components, particularly in the form of calcium hydroxide. The effluent gases from the calcination of limestone have been used to react with calcium hydroxide to provide precipitated calcium carbonate. The term "precipitated calcium carbonate" generally applies to the commercial types of calcium carbonate produced by chemical means. Paper making processes, which result in the production of discharges having a high level of calcium hydroxide, are sometimes closely associated with the output from a limestone processing plant. The availability of effluent carbon dioxide rich gas is dependent upon the operation of the limestone producing operation at the time that the gas is desired. All methods of production of precipitated calcium carbonate depend upon bringing together calcium ions and carbonate ions under controlled conditions followed by separating, drying and disintegrating or powdering the calcium carbonate so produced and such methods, heretofore, have been dependent upon the use of gaseous carbon dioxide.
A principal use of precipitated calcium carbonate is in the paper making industry as a filler. Paper, as laid down from chemical pulp, does not have the opacity which is necessary for a good print quality where two sides are printed. Paper from ground wood pulp, such as news print, has course fibers which provide a large enough surface area to permit printing without print through. High quality papers from chemical pulping, must be filled to overcome the translucency of these fibers.
The most common fillers are mineral pigments, of which calcium carbonate is a principal pigment. Other pigments include clay, titanium dioxide, hydrated alumina and talc. Calcium carbonate is used in those paper making systems that are neutral or slightly alkaline because it reacts with acid. Among the commonly used fillers, titanium dioxide provides the highest brightness but is extremely expensive. Calcium carbonate is a close second to the titanium dioxide in brightness. One commercial manufacturing process for supplying precipitated calcium carbonate for the paper industry starts with limestone or any naturally occurring calcium carbonate. The natural calcium carbonate contains impurities which are partially removed by calcining and completely removed by later processing steps. The calcining yields calcium oxide and carbon dioxide at the decomposition temperature in the kiln. Limestone is calcined in a kiln to produce quicklime and kiln gases. The quicklime is slaked with water, screened and diluted with addition water to provide milk of lime containing about 70-80 grams of calcium oxide per liter. The kiln gases containing 30% or more carbon dioxide are purified by scrubbing and are then fed into the lime suspension. Contact between liquid and gas is effected by spraying the lime suspension through the carbonating chamber and by use of paddle-type agitators installed at the bottom of the chamber. This system of manufacture allows considerable latitude in control of temperature, concentration of reactants and speed of reaction; factors that may influence the particle size distribution of the calcium carbonate produced and the properties dependent thereon.
The result is a high purity calcium carbonate whose type of crystal and particle size can be controlled by the reaction conditions in the carbonation steps. Theses conditions are optimized for the characteristic most desirable for paper coating.
As indicated above, precipitated calcium carbonate has also been produced as a by-product of the paper making industry by treatment of caustic discharges with gaseous carbon dioxide. Such treatment of the discharge from paper making processes has relied upon the presence of a large amount of gaseous carbon dioxide, usually from a limestone treatment facility, or by vaporization of liquid carbon dioxide. In an integration of a limestone kiln with a paper mill, the clean up of the gases from the limestone kiln is usually not economically justifiable and the resulting precipitated calcium carbonate can be contaminated, resulting in a lower grade of calcium carbonate less desirable for use in the filling of paper. In any event, it has been thought necessary to provide a relatively high quality source of gaseous carbon dioxide to produce the finer grades of precipitated calcium carbonate. The relationship between particle size and brightness is clearly evident with the finer particle sizes often imparting greater brightness to the paper.
In accordance with the present invention, a supplementary source of carbon dioxide is provided for use in the manufacture of precipitated calcium carbonate from the discharge processes of a paper making process. It has been previously proposed to use liquid carbon dioxide in the manufacture of precipitated calcium carbonate by a process involving vaporization of the liquid carbon dioxide prior to contacting the resultant gaseous carbon dioxide with calcium hydroxide to produce the calcium carbonate. Such vaporization was thought to be necessary due to the exothermic reaction between calcium hydroxide and carbon dioxide. In accordance with the present invention, it has been discovered that liquid carbon dioxide can be injected directly into a calcium hydroxide solution to produce precipitated calcium carbonate. The process of the present invention provides benefits of power consumption savings involved in the necessity to vaporize liquid carbon dioxide into gaseous carbon dioxide. A further benefit is that production is not limited by concentration and quantity of lime kiln stack gas which may be available. Supplemental liquid carbon dioxide provides the capability to produce precipitated calcium carbonate as needed on demand in a paper making process. Existing reaction vessels may be used without substantial modification to provide flexibility and increased production.
Accordingly, it is the principal object of the present invention to provide a method for carrying out an exothermic chemical reaction with liquid carbon dioxide in an aqueous environment.
It is another object of the present invention to provide a method for the manufacture of precipitated calcium carbonate utilizing liquid carbon dioxide.
It is further object of the present invention to provide a method for producing precipitated calcium carbonate by direct injection of liquid carbon dioxide to provide a precipitated calcium carbonate product with improved properties for use as a filler in paper making processes.
These and other objects of the invention will become more apparent from the following detailed description and the accompanying drawings wherein: